The present invention relates to optical fibers prepared with a primary coating composition including a monomer with a pendant hydroxyl functional group and a method of making optical fibers that contain such coatings.
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
The fibers used in local networks are directly exposed to harsh conditions, including severe temperature and humidity extremes. Since prior coatings for optical fibers did not perform well under such adverse conditions, the need existed for the development of higher performance coatings to address the wide and varied temperature and humidity conditions in which fibers are employed. Specifically, these coatings possessed thermal, oxidative, and hydrolytic stability which is sufficient to protect the encapsulated fiber over a long life-span (i.e., about twenty-five or more years).
Optical fibers typically contain a glass core, a cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the cladding and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers and cause attenuation of the light which is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained throughout the fiber""s lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) which is lower than the foreseeable lowest use temperature. This enables the primary coating to remain soft throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating.
It is important for the primary coating to have a refractive index which is different (i.e., higher) than the refractive index of the cladding. This refractive index differential between the cladding and the primary coating allows errant light signals to be refracted away from the glass core.
Finally, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. Moisture resistance is essential, because moisture also affects the adhesion of the primary coating to the glass. Poor adhesion can result in various sized delaminations which may lead to microbending and which can be significant sources of attenuation in the optical fiber.
Therefore, the need remains for improved coating compositions for glass fibers. The present invention is directed to overcoming this deficiency in the art.
One aspect of the invention relates to an optical fiber which includes a glass fiber and a primary coating encapsulating and in contact with the glass fiber. This primary coating is the cured product of a polymerizable composition that includes a monomer with a pendant hydroxyl functional group which can bond with a surface of the glass fiber and is present in an amount effective to promote adhesion.
Another aspect of the invention relates to a method of making an optical fiber in accordance with the present invention. This method involves providing a glass fiber, coating the glass fiber with a polymerizable primary coating composition that includes a monomer with a pendant hydroxyl functional group which can bond with a surface of the glass fiber and is present in an amount effective to promote adhesion, and polymerizing the composition under conditions effective to form a primary coating over the glass fiber.
By employing a primary coating composition of the present invention, it is possible to enhance the adhesive property of the primary coating on an optical fiber, thereby increasing pullout values. As a result, the occurrence of delaminations can be reduced or eliminated. Further, the useful life of optical fibers can be enhanced and the quality of signal transmission (i.e., minimal attenuation) over the lifetime of the optical fiber can be maintained.